1. Field of Invention
This invention relates to a process for the removal of radioactive materials from aqueous solutions, and particularly the removal of relatively low concentrations of materials as might occur naturally or after highly contaminated waste water has been treated.
2. Description of the Prior Art
The Water Pollution Control Act and Safe Drinking Water Act have imposed specific requirements on the quality of water discharged into streams and served to the public as potable water. As need increases, the regulations promulgated under both of these pieces of legislation will ultimately place more stringent requirements on both waste discharges and potable water purveyors. These requirements will include limits for radioactive materials, including uranium.
The primary sources for radioactive materials carried in solution are mining operations and certain nuclear fuel producing facilities. As more and more attention is given to the quality of life and technology's impact on society, as well as plant and animal life, the treatment of even these relatively low concentrations of radioactive materials becomes more important. For example, the Environmental Protection Agency has given the Colorado Department of Health a recommendation that dissolved uranium not exceed 0.015 mg/l. This is substantially less than concentrations 0.18 to 2.3 mg/l occurring in natural flowing streams as a result of uranium mining operations.
The prior art has directed much attention to the reduction of high level concentration of radioactive materials and waste products resulting from the processing, production and decontamination of nuclear fission materials. Precipitation of the radioactive material from the solution is an often used part of these processes. U.S. Pat. No. 2,854,315 to Alter et al describes the treatment of a nitric acid solution contaminated by high level amounts of radioactive material by the addition of alkali metal hydroxides, which precipitates the radioactive wastes for later disposal. The removal of fission products, primarily from sea water, by precipitation is described in U.S. Pat. No. 3,013,978, to Rosinski. The process described relies on a mixture of precipitants, such as manganese dioxide or chromic oxide and ferric hydroxide, and the pH is allowed to vary widely. U.S. Pat. No. 2,766,204 to Lowe, specifies that the pH be held within a relatively narrow range of 3.6 to 4.4, by addition of an alkali hydroxide or an alkali carbonate, during a FeS precipitation process, which is conducted at low temperatures, about 0.degree. C. The Lowe process also suggests initially adjusting the solution to a pH of 1.0 with nitric acid.
Direct precipitation of radioactive nuclear fission products from water by the addition of water soluble titanium compounds is disclosed in U.S. Pat. No. 3,330,771 to Komatsu et al. The dissolved radioactive cations along with the colloidal radioactive substances are coprecipitated and removed from solution.
Lime is utilized in a declading process to precipitate fluorides and neutralize sulfuric acid in Detilleux et al, U.S. Pat. No. 3,557,013. In Detilleux, the high level wastes remain in the solution after the fluorides are precipitated out by adding lime, evaporation of the remaining solution over a long period of time leaves a solid mass of radioactive wastes which are accumulated for further disposal.
Lime is also used to precipitate out florides in Crossley, U.S. Pat. No. 3,961,027. The remaining solution is distilled and then treated by cationic ion exchange leaving water containing small amounts of uranium, fluoride and ammonia which can be recycled to react with UF.sub.6 or combined with ammonium hydroxide distillate and then treated with additional concentrated ammonium hydroxide to form a solution of desired NH.sub.3 content for use in precipitating ammonium diuranate.
Even more complex processes are used to treat waste water solutions, as disclosed in U.S. Pat. No. 3,008,904, to Johnson et al, wherein a phosphate, silicate or borate is added to the solution which is in turn entrained by a steam spray into a space heated to between 250.degree. and 400.degree. C. The residue is calcined and the water vapor and gaseous products separated.
Treatment of the water resulting from uranium ore processing, by evaporation and crystallization is shown in U.S. Pat. No. 3,988,414 to Klicka et al. Naturally occurring radioactive materials, including ruthenium and iodine, are mechanically filtered out of water to be used for drinking purposes, in an apparatus described in U.S. Pat. No. 3,405,050, to Bovard et al, embodying a series of stacked filters, one of the filters containing a resin for ion exchange with the radioactive materials.
Bovard also describes the prior art use of lime and sodium carbonate in water treatment. Lime is described as a flocculant which can help coagulate unspecified radioactive elements. Lime and sodium carbonate are reported to have experienced "some success" in removal of radioactive elements in water treatment work.
Ion exchange has been widely used in radioactive waste water treatment. It is effective, but involves costly resins and decreases in efficiency as competing nonradioactive salts increase in concentration. Reverse osmosis, a process utilizing a membrane and high pressure energy intensive pumps to create a brine product, is likewise expensive and not readily adaptable to large city water treatment plants.
Few of the above described processes are directly applicable to small amounts of naturally occurring radioactive materials dissolved in aqueous solution. The processes that involve precipitation are generally directed at expensive less readily available, precipitating agents.
Conventional city water treatment processes do use lime as a coagulant during the treatment of the water. Relatively small amounts of lime, less than 10 mg/l, are added to form colloids, which have the physical property of adhesion. Flocculation of the suspended colloids allows various colloidal massesto agglomerate or adhere to each other and eventually settle to the bottom of a tank. This process is primarily directed to matter which is suspended in the water to be treated, rather than dissolved materials.
Other water treatment works rely on lime or lime combined with soda ash to soften, remove hardness, and remove divalent cationic elements such as Ca.sup.++ and Mg.sup.++. These treatment processes add soda ash and/or lime for purposes of precipitating the divalent cations from solution as Mg(OH).sub.2 and CaCO.sub.3. The lime and soda ash treatment process is applicable for the removal of certain fission products, particularly strontium. However, to accomplish these removals very high chemical doses are required (300 mg/l lime). Uranium is not a fission product, and remains in solution, unaffected by the lime-soda ash softening process.